1. Field of the Invention
The present invention relates to an apparatus for manufacturing silicon single crystals by the Czochralski method.
2. Description of the Prior Art
The silicon single crystal pulling process according to the Czochralski method has been used in the past and it has become a practically complete technique. However, where the specification is severe, the yield of usable wafers will be reduced to less than 50% due to the maldistribution of the dopant and oxygen.
As an effective means of solving such a problem, the prior art shows a method in which a silicon starting material is continuously fed into a crucible having a double structure, thereby maintaining the surface level of the molten material constant (Laid-Open Patent No. 40-10184). In particular, the manufacture of high quality granular polycrystal silicon has recently become possible and it has been considered relatively easy to feed this granular silicon to the molten material continuously at a constant feed rate, thus leading to the publication, of some inventions and a paper (inventions of Laid-Open Patents No. 58-130195 and No. 63-95195 and Laid-Open Utility Model No. 59-141578 and the paper Ann. Rev. Mater, Sci, 1987, Vol. 17, P273-278).
The inventions disclosed in these publications, etc., are of the type employing a silica glass crucible of the double structure. However, as particularly pointed out in the publication of Laid-Open Patent No. 62-241889, there is the problem of the solidification of the melt which tends to occur at the contacting portion between the inner surface of the inner crucible and the molten silicon surface, thus making it difficult to decrease the furnace temperature to the temperature required for the stable growth of a single crystal. If the pulling of a single crystal is effected by keeping the temperature of the molten silicon high so as to prevent the occurrence of solidification, not only is the rate of solidification decreased but also the occurrence of dislocation frequently takes place, thus making it impossible to stably manufacture a single crystal. Also the invention disclosed in Laid-Open Patent No. 61-36197 employs a crucible of the double structure as well as a heat insulating material arranged above the outer material melting section and a separate heater arranged at the bottom of the crucible so as to promote the melting of the starting material supplied. However, this invention also involves no means for preventing the dissipation of heat from the contacting portion between the inner surface of the inner crucible and the molten silicon surface so as to prevent the occurrence of solidification. In addition, the heating by the heater at the bottom of the crucible decreases the temperature of another heater arranged along the side of the crucible thus promoting the occurrence of solidification.
In the manufacturing method of the above-mentioned type using the partition of the inner crucible (hereinafter referred to as a partition), particularly, the method of manufacturing large-diameter silicon single crystals of 12 to 30 cm, the heat dissipation from inside the partition tends to decrease the temperature of the molten material inside the partition, particularly the molten material contacting with the partition. This is considered to be caused by the fact that the material of the partition is transparent silica glass having an emissivity considerably greater than that of the molten silicon and a great amount of heat is dissipated from the partition to the water-cooled cover above the crucible. Also, due to the crucible being of the double structure, the convection of the molten silicon is limited and the temperature inside the partition has a lesser tendency to rise.
Moreover, while the temperature of the molten silicon inside the partition (crystal growing section) must be maintained just above the melting point of silicon in order to ensure the growth of a single crystal, due to the previously mentioned phenomena, there is caused the problem of the solidification starting at the contacting portion between the partition and the surface of the molten material inside the partition.
However, the previously mentioned conventional techniques have not been provided with any means for preventing the occurrence of such solidification.